For example, when gases such as a flammable gas, a toxic gas, and vapor of an organic solvent are leaked from piping, tank, and the like, the leakage needs to be dealt with early. In addition, to determine the danger, the concentration-thickness product, preferably, the concentration of the leaked gas leaked into a space needs to know. Therefore, devices for measuring a gas such as the leaked gas are demanded. As such devices, for example, Patent Literature 1 discloses a technique for obtaining a concentration-thickness product, and Patent Literature 2 discloses a technique for correcting a concentration-thickness product on the basis of a difference between a predetermined average temperature and a background temperature.
Here, in the technique disclosed in Patent Literature 1 described above, an amount of infrared rays is obtained at two points A and B where background temperatures through the leaked gas are different, by an infrared camera, a value of the concentration-thickness product ct is assigned in the following equation 1, and the concentration-thickness product ct by which both the sides of the following equation 1 become equal is obtained as the concentration-thickness product ct of the leaked gas (second sight method).PB−PA=ε∫exp(α(λ)ct)S(λ)[B(Tback_B,λ)−B(Tback_A,λ)]dλ  (1)
Here, PA is the amount of infrared rays observed by the infrared camera at the point A, B(Tback_A, λ) is the amount of background radiation infrared rays at the point A (Tback_A is the background temperature at the point A and λ is the wavelength), PB is the amount of infrared rays observed by the infrared camera at the point B, (Tback_B, λ) is the amount of background radiation infrared rays at the point B (Tback_B is the background temperature at the point B), S(λ) is the transmittance of an optical system, ct is the concentration-thickness product of the gas (c is the concentration and t is the thickness), c is the background emissivity, and α(λ) is the gas absorption rate. The integral ∫ is executed over the wavelength range of observed infrared rays.
By the way, in the technique disclosed in Patent Literature 1 described above, if the difference between the background temperature (the amount of background radiation infrared rays) and the leaked gas temperature is small, a signal regarding the background temperature (the amount of background radiation infrared rays), which is obtained by the infrared camera, becomes small and the signal-to-noise ratio (SN ratio) is decreased. As a result, a large error is included in the concentration-thickness product. For this reason, although the concentration-thickness product may be corrected by the technique disclosed in Patent Literature 2, complicated correction calculation is executed in Patent Literature 2, and therefore information processing of the correction calculation takes time, and a large difference is caused between observation timing by the infrared camera and calculation output timing of the concentration-thickness product, resulting in losing real-time properties. Particularly, the leaked gas needs to be dealt with as soon as possible, the real-time properties are important for calculation of the concentration-thickness product, which serves as an index for determining if it is necessary to deal with the leaked gas as soon as possible.